Laser absorption spectrometry (LAS) may be used to assess the concentration or amount of a species in gas phase by absorption spectrometry. As well as quantitative assessments of the amounts of different atoms and molecules in the gas phase, LAS can also be used for isotope ratio measurements based on absorption by isotopologues of the same molecule. Such application of LAS is called Isotope Ratio Optical Spectrometry (IROS). For example, 13C:12C and 18O:16O isotope ratios in a sample of CO2 gas may be assessed.
In LAS, the light emitted from a laser source is passed through the gas to be analysed to a detector that measures the intensity of received laser light. The wavelength of the laser light is scanned across the absorption peaks of an atomic or molecular species in the gas. An absorption peak of a species occurs at a wavelength (a peak position) at which the light is absorbed by the species. A reduction in the measured signal intensity at a characteristic peak position (i.e., at a characteristic wavelength) may indicate the presence and/or concentration of a particular species. In the case of IROS, each isotopologue of the species has at least one characteristic peak position. A reduction in the measured signal intensity at a characteristic peak position may indicate the presence of a particular isotopologue, and the extent of the reduction may be used to determine the concentration of that isotopologue and/or an isotope ratio in that species. For example, CO2 isotopologues 12C16O16O, 13C16O16O and 12C18O16O each have different absorption peaks at specific wavelengths due to quantum mechanical rotational-vibrational states (i.e., each isotopologue absorbs different wavelength light). Measuring the different absorption peaks of two or more isotopologues can be used to determine an isotope ratio such as 13C:12C or 18O:16O in the CO2.
The laser source used in LAS may comprise at least one laser diode. The wavelength of light emitted from the laser diode may be altered, or tuned, by changing at least one laser parameter. The wavelength of light emitted from the laser diode may therefore be scanned across a range of wavelengths by changing at least one laser parameter. The laser parameters may comprise the laser diode temperature and/or the injection current, also termed drive current, to the laser diode.
For each wavelength to which the laser is tuned when being scanned across the absorption peaks of a species, it is preferable for the laser to operate at a single optical frequency (i.e., at any single moment, the laser operates at a single optical frequency). Furthermore, it is also desirable that as the wavelength of the laser light is scanned across the absorption peaks, the wavelength is tuned (i.e., increased or decreased) continuously and predictably. In practice, this may not be possible due to mode hopping and/or multimoding, as explained in the document ILX Lightwave, Mode Hopping In Semiconductor Lasers, Application Note #8 (2005), which is available at http://assets.newport.com/webDocuments-EN/images/AN08_Mode_Hopping_Laser_Diode_IX.PDF. Multimoding is where the laser diode outputs multiple optical frequencies associated with different resonator modes. Mode hopping is where the laser diode exhibits sudden, unpredictable jumps of the wavelength associated with different resonator modes.
To achieve single-frequency operation and continuous, predictable wavelength tuning, both multimode operation and discontinuities in wavelength caused by switching between different resonator modes should be avoided. During initial calibration of a laser absorption spectrometer, the operating point of the laser diode, which is defined by the laser parameters, may be set with this in mind.
Typically, an optical spectrum analyser is used to measure the wavelength of the laser light and judge if the laser diode is operating at a single optical frequency. A laser parameter map is generated by varying the laser parameters, measuring the wavelength of the laser light at each combination of parameters and judging if the laser diode is operating at a single optical frequency at each combination of parameters. Stable regions of the laser parameter map may then be identified and the operating point of the laser diode (i.e., the particular laser parameter values around which the laser will operate) is chosen to be in the centre of a stable region of the laser parameter map.
However, the stable regions may change as a laser ages. Consequently, over time, the stable regions on the laser parameter map may move such that the operating point of the laser diode is no longer in the centre of the stable region and may be towards a discontinuity and/or a region of multimoding. If this happens, during operation of the laser absorption spectrometer, as the wavelength of the laser light is varied across the absorption peaks of a species, at times the laser light might not operate at a single optical frequency and/or the wavelength might not tune continuously and predictably. This may result in errors and inaccuracies in measurements from the laser absorption spectrometer.
Consequently, it is desired to have an efficient procedure for re-optimizing the operating point of a laser built into a laser absorption spectrometer with respect to stability.